Fentanyl is a synthetic narcotic analgesic commonly usually used in anesthesia. Little is known about how critically ill patients metabolized and eliminate fentanyl. Twenty patients will be studied over a 3 year period to determine the pharmacokinetic paramters of fentanyl. Quantitative analysis of plasma samples will be performed in the Clinical Pharmacokinetics Research Laboratory. The method of analysis is a gas chromatographic system which is both sensitive and selective. Data analysis will be done in the same laboratory.